1. Field of the Invention
The present invention relates to an apparatus and a method for performing a heat treatment (e.g., a baking treatment) on a substrate, such as a mask substrate (reticle substrate), a semiconductor wafer, or a glass substrate for a liquid crystal display, and more particularly to a technique for controlling a temperature of the surface of the substrate.
2. Description of the Related Art
A baking treatment, which is one sort of the aforementioned heat treatment, is carried out after the substrate is coated with a resist solution, or before or after the substrate is coated with a developing solution, for example. The baking apparatus is incorporated into a coating-and-developing system.
Referring to FIG. 12, a conventional heat treatment apparatus is provided with a hot plate 12 in which a heater 11 is embedded. A substrate G is heated while it is placed on the hot plate 12 via proximity pins 13. The proximity pins 13 separate the back surface of the substrate G from the upper surface of the hot plate 12 by a small distance (e.g., 0.5 millimeters) in order to prevent particles from adhering to the back surface of the substrate G. In general, the temperature control of the substrate G is carried out by measuring the temperature of the surface area of the hot plate 12 by the temperature sensor 14, and by regulating electric power supplied to the heater 11 by a PID (Proportional Integral Differential) controller 15 based on the deviation of the measured temperature from the target temperature.
WO 00/51170 discloses a baking apparatus designed for improving in-plane temperature uniformity over the substrate surface. The apparatus includes a hot plate, which is heated by plural heating elements that can be separately controlled. The temperature of the heating elements is measured and the heating process is controlled by a PID controller. The temperature of the substrate surface facing away from the hot plate is locally measured by a temperature measuring device such as an infrared camera arranged above the substrate, and the temperature distribution over the substrate surface is determined according to the measured temperatures. Set values for the temperature of the individual heating elements are determined and transmitted to the PID controller. The PID controller controls the individual heating elements based on the set values and the measured temperature of the surface areas of the hot plate corresponding to individual heating elements.
JP11-8180A discloses a baking apparatus provided with an infrared temperature sensor which measures the temperature distribution over the entire surface of a resist film coated on a substrate. The temperature of the substrate heated by a heater block is controlled based on the temperature distribution measured by the infrared temperature sensor.
JP11-74187A discloses a temperature control method for a substrate heat treatment apparatus. The substrate is heated by a heater embedded in a hot plate supplied with a fixed electric power during a substrate temperature rising stage. After the substrate temperature reaches heat treatment temperature, the electric power supplied to the heater is controlled by a PID controller.
However, the above conventional techniques have the following problems.
A change in the substrate temperature follows a change in the heater temperature with a time delay. Notably, with a thick substrate such as a mask substrate, the delay time is long. If the substrate temperature is controlled in a PID control mode as disclosed in WO 00/51170, when rapidly raising the temperature of such a thick substrate, the substrate temperature tends to overshoot the target value. Thus, the substrate temperature rising rate must be low. The temperature distribution over the substrate surface during the temperature rising stage is unavoidably wide. Accordingly, if the temperature rising rate is low, the distribution of the time integral of the temperature over the substrate surface becomes considerably wider, resulting in deterioration of in-plane uniformity of the baking treatment.
When the set temperature of each heating element is corrected based on the temperature distribution of the substrate, the set temperature must be significantly changed. However, at the late phase of the temperature rising stage, in other words when the substrate temperature is close to the target temperature, if the set temperature is significantly changed, the substrate temperature becomes unstable or fluctuates. This results in deterioration of in-plane temperature uniformity over the substrate surface, thus resulting in non-uniformity of the thickness, quality and line width of the resist pattern.
Accordingly, the temperature control of WO 00/51170 is not suitable for use in the substrate temperature rising stage. Recently, the layer thickness of semiconductor devices has become thicker, and the line width of the circuit pattern has become narrower. Thus, high in-plane temperature uniformity is required in the substrate temperature rising stage. However, the temperature control of WO 00/51170 does not meet such requirement.
The apparatus of JP11-8180A also controls the heating operation based on the temperature distribution data, which is effective for use in the stable temperature stage, but is not suitable for use in the temperature rising stage.
The apparatus of JP-74187A does not have any means for controlling electric power supplied to the heater, and thus can not respond to the change in actual substrate temperature due to abnormalities such as change in air flow in the processing vessel, or variation in substrate thickness.